The font is probably "Sans Serif", you can check
which font this expands to with

fc-match sans

When Arial/Albany AMT are installed this will probably expand
to Arial or Albany AMT on your system, when they are not
installed it will expand to a font which renders considerably
bolder at the same point size (probably "DejaVu Sans" and
probably your have just the dpi where "DejaVu Sans" switches to
2 pixels stem width at the default point size. Fontsizes differ
somewhat for the same point size for fonts of different makers,
DejaVu is a bit bigger at the same point size than Arial.
If you increas the size a little bit, Arial will also switch
to 2 pixel stem with. This is obvious when the byte code interpreter
is used, with the autohinter (hintstyle medium forces the autohinter)
the blurriness prevents a obvious transition from 1 pixel stem width
to 2 pixel stem width).